linking to literacy · because bodies of water act as heat sinks. coastal cities are generally...

316 Chapter 12 • Water, Weather, and Climate NEL

12.1 Surface Water and ClimateSummers in Ontario can be rainy and wet. When this happens, the media oft en show images of swollen rivers and fl ooded farm fi elds (Figure 1). In other years, unusually dry summers result in drought. It seems that every year, some weather record is broken: “Hottest day in June” or “Most snow in one day for a century.” Many other countries are also experiencing record-breaking weather.

Weather and ClimateWhat is the diff erence between weather and climate? Weather is a set of environmental conditions that changes from day to day. A description of a day’s weather might include the temperature, whether it is sunny or cloudy, and what kind of precipitation is falling. Th e weather in an area changes during a day. Climate, on the other hand, is a set of environmental conditions averaged over 30 years or more. It does not describe any particular event at any particular time.

Consider Timmins in northern Ontario. We can describe its February climate as cold. Th e average daytime high temperature is –7.5 °C, and the average overnight low is −21.3 °C. Th e average snowfall for the month is 40.6 cm. Th e weather on February 20, next winter, however, might be a little diff erent. Th ere could be a daytime high of 5 °C and an overnight low of –5 °C, and no snow. You can use climate data to make a prediction about the conditions at a certain location and at a certain time of year. Th e actual day-by-day weather that occurs, however, might be quite diff erent.

weather: the day-to-day environmental conditions in a given place at a given time; includes temperature, cloud cover, wind speed, and precipitation

climate: an average of weather conditions in an area over 30 years

Figure 1 Extreme weather, such as intense rainfall, can cause property damage.

Making ConnectionsWhen you read, you can make connections with the text. There are three types of connections that you can make: 1. Text-to-text: Read the fi rst

paragraph on this page. How does it relate to the comic strip on the previous page?

2. Text-to-self: What extreme weather have you experienced? Have you been in a tornado, fl ood, or other type of dangerous weather? How does your experience relate to information on this page?

3. Text-to-world: How does the text on this page relate to places, people, or events in the world? Perhaps you have seen something about this topic on the news, on the Internet, or in a newspaper.

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Sci8_UnitD_Chap12.indd 316Sci8_UnitD_Chap12.indd 316 10/24/08 12:45:35 PM10/24/08 12:45:35 PM

Sun

energy radiatedfrom Earth(day and night)

energy radiatedfrom the Sun

31 °C 23 °C

gravel water

12.1 Surface Water and Climate 317NEL

Causes of Temperature DifferencesEnergy from the Sun (solar energy) drives the evaporation of water in the water cycle and also heats Earth’s surface. What happens to Earth’s surface at night, when the Sun disappears below the horizon? Earth cools down as it radiates more energy than it absorbs (Figure 2). Th e change from daytime heating to nighttime cooling is so dramatic on most planets that they cannot support life.

Heat CapacityWhy does Earth support life better than other known planets? Part of the answer lies in the substances that make up Earth’s surface: water, soil, and rock. An important property of these substances is their heat capacity. Heat capacity is a measure of how much thermal energy (heat) a substance requires to increase its temperature by a certain amount. Th e higher the heat capacity of a substance, the more thermal energy it can store. Soil and rock have lower heat capacities than water. Th is means that soil and rock require less thermal energy than water to increase in temperature (Figure 3).

Heat capacity also includes the amount of thermal energy that a substance releases. If the light source is removed from the gravel and water in Figure 3, they would begin to release their stored thermal energy to the surrounding air. Th e gravel, however, would release its energy more quickly. Gravel has a lower heat capacity than water. Because of this, the gravel would cool down faster and heat the surrounding air faster. However, once the gravel has cooled, it would no longer be able to heat the air, and the air would begin to cool. Th e water would heat the air more slowly, but for longer. Water can store energy for a long time, even if the air above it cools at night, or if the seasons change.

Earth’s atmosphere is another important part of what makes Earth so diff erent from most planets. Th e atmosphere traps some of the energy radiated from Earth. Th is helps keep Earth warmer than it would be otherwise.

heat capacity: a measure of the amount of thermal energy needed to raise the temperature of a substance by a certain temperature interval; a measure of how much thermal energy a particular object can store

Figure 2 Earth radiates energy at all times. As it rotates, the side of Earth facing the Sun absorbs solar energy and Earth warms. The side of Earth turned away from the Sun does not receive solar energy.

Figure 3 If equal amounts of water and gravel start off at the same temperature and are exposed to the same amount of thermal energy, the gravel will heat up to a higher temperature than the water because gravel has a lower heat capacity.


cool air sinks

warm airpushed upward

land

sea

318 Chapter 12 • Water, Weather, and Climate NEL

Heat Sinks and ClimateAnything that absorbs thermal energy without becoming much warmer is called a heat sink. Substances with high heat capacities, such as water, are good heat sinks. Oceans can absorb a lot of energy from the Sun without evaporating. In fact, the temperature of oceans remains fairly constant.

Water absorbs energy when sunlight hits the surface of the water. Water is continually moving and mixing. Th is moves deep, cooler water to the surface, where it absorbs more energy. In this way, energy is transferred throughout the entire body of water. Th is is what prevents ocean water from heating up.

Th e temperature of land increases more on a sunny day than does the temperature of nearby water. Both land and water can warm the air above them, but the lower heat capacity of rock and soil means that the air above the warmed land heats up faster than the air above the water. When air becomes warm, it becomes less dense. As cooler, heavier air sinks, it pushes the warmer, lighter air upward (Figure 4). Th is diff erence in density causes air movements called convection currents, which we feel as wind.

Land gives up stored thermal energy much more rapidly than water. When the Sun goes down—or when the days get shorter in the winter—land cools more rapidly than does the water.

Consider how a body of surface water stores energy and releases it slowly as the surrounding temperature falls. Table 1 compares the temperatures of two coastal cities and two inland cities (averaged over many years).

heat sink: matter that absorbs thermal energy but does not change state or signifi cantly increase in temperature

convection: the transfer of thermal energy from one part of a gas or liquid to another by a circulating current of faster-moving and slower-moving particles

Figure 4 When the land is warmer than the water, the air over the land is less dense than the cooler air over the water. The denser air above the water fl ows down and toward the land, causing a sea breeze.

Table 1 Climate Data for Four Canadian Cities

Victoria Winnipeg Timmins St. John’s

average January minimum temperature (°C)

2 −22 −24 −7

average July maximum temperature (°C)

20 27 24 21


NORTHWESTTERRITORIES NUNAVUT

YUKON

BRITISH COLUMBIA

ALBERTA

SASK

ATCH

EWAN

MANITOBA

ONTARIO

NOVA SCOTIA

PRINCE EDWARDISLAND

NEWFOUNDLANDAND

LABRADOR

NEWBRUNSWICK

Victoria

TimminsWinnipeg

St. John’sQUEBEC

12.1 Surface Water and Climate 319NEL

You would expect each of the cities in Table 1 to receive the same amount of energy from the Sun throughout the year (Figure 5). However, you can see that the inland cities, Winnipeg and Timmins, are colder in the winter and hotter in the summer than the coastal cities.

Scientists have taken temperature measurements of soil and water over many years. Th ese measurements indicate that only land near the surface experiences heating and cooling; farther underground, the temperature remains constant. Temperature changes occur in water to a much greater depth than on land because the water’s surface is stirred up by wind, waves, and ocean currents. Th ese factors help to mix warm surface water with cooler deep water.

Land near water has a milder climate than land far from water because bodies of water act as heat sinks. Coastal cities are generally warmer in the winter and cooler in the summer than inland cities. Southwestern Ontario has a more moderate climate than neighbouring regions to the west and east of the Great Lakes because the Great Lakes act as heat sinks. Th ey absorb thermal energy during the summer and gradually release it during the winter.

Figure 5 Victoria, Winnipeg, Timmins, and St. John’s are all approximately the same distance from the equator. However, the four cities have very different climates.

CHECK YOUR LEARNING

1. What is the difference between weather and climate?

2. Explain in your own words how the heat capacity of water makes any body of water a good heat sink.

3. Describe how warm water at the surface of a lake or ocean mixes with deeper, cooler water.

4. Why do areas close to large bodies of water have milder climates than inland areas?

C

Making InferencesSometimes a text has an inferred meaning. “Inferred” means that a text says something, but also means something more. The text here does not tell you about the temperature of cities like Toronto or Halifax. Think about what you know about these cities. One is on the shore of the Great Lakes and the other is on the East Coast. What have you read in the text to infer that Toronto and Halifax enjoy milder climates than many Canadian cities?

What other Canadian cities are you familiar with? What can you infer about their temperatures based on their coastal or inland locations?

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